DXO has come out with tests on the Canon 35mm f2 IS. I compared it to the Sigma 35mm f1.4 ART, Canon 28mm f2.8 IS and Canon 50mm f1.4 (the last two I have).

DXO only gives a single number for sharpens (Mpix) which is to me only a small single bit of data. I like full range MTF like lenstip.com does. Anyway, it does give a number to compare. The Mpix numbers on the Canon 5D MK II for the lenses above are:

Canon 28mm f2.8 IS: Mpix=15P

Canon 50mm f1.4 HSM: Mpix=15P

Sigma 35mm f1.4 ART: Mpix=17P

Canon 35mm f2 IS: Mpix-=17P

Since I own and have extensively used and tested the first two I can say that according to what I have seen the two lenses are very close in sharpness (I can't tell the difference). Initially I was finding DXO Mark data to be suspicious but after a lot of having looked at may cameras and compared cameras and lenses I own (or have owned) I am very confident now in their data. I especially like the tonal range graphs to compare cameras (very useful to me).

The comparison between the Sigma 35mm f1.4 and Canon 35mm f2 indicate the same sharpness. That conflicts with what lensrentals found. Woud like to see other data like Photozone and SLR to compare all of the above (not all exist at this time).

Looking forward to the Canon 50mm f1.4 IS. If Canon is able to make one that is sharper in the f1.4-f2 range than the current one that would be great. Otherwise I probably wouldn't bother. I expect Sigma will be coming out with an ART version in the near future.